Wireless communication systems that utilize digitally coded communication signals are known in the art. One such system is a direct sequence Code Division Multiple Access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 2000 (TIA IS-2000) herein after referred to as IS-2000. In accordance with IS-2000, the coded communication signals used in the DS-CDMA system include signals that are transmitted in a common channel, typically a 1.25 MHz bandwidth common channel, between mobile stations (MS) and base transceiver stations (BTS) located at the base sites (BS) of a wireless communication system.
A digital wireless communication system is a complex network of elements and their interconnections and protocols. Typically elements include (1) a radio link to the mobile stations (e.g., cellular telephones), which is usually provide by at least one and typically several base stations, (2) communication links between the base stations, (3) a controller, typically one or more base station controllers or centralized base station controllers (BSC/CBSC), to control communication between and to manage the operation and interaction of the base stations, (4) a call controller or switch, typically a call agent (i.e., a “softswitch”), for routing calls within the system, and (5) a link to the land line or public switch telephone network (PSTN), which is usually also provided by the call agent.
Optimization of performance efficiency of a wireless communication system is generally accomplished through fault detection testing. Of particular importance in a wireless communication system is the quality of a digital voice channel path. The typical digital voice channel path may include the over-the-air interface between a source mobile station and a destination mobile station as well as all of the infrastructure used to connect the source mobile station to the destination mobile station, for example the BTS, the transcoder in the CBSC, and the MSC. Various types of fault detection testing have been used in an attempt to detect problems in the digital voice channel path. The most well known testing method, commonly referred to as the analog system audio test, or SALT, was developed originally for use in analog communication systems. The SALT test utilizes special test equipment to inject a single-frequency audio tone into a previously established voice call. The single-frequency audio tone is looped back to the sending equipment where its frequency and audio level are measured. The test is considered to be successful (indicating a working channel) if the frequency and audio level are reasonable close to the frequency and audio level of the injected tone. SALT however, has a number of problems when applied to a digital based wireless communication system. First, a single-frequency audio tone is not reproduced well by the digital infrastructure of a wireless communication system. As expected, vocoders in the call channel distort a single-frequency audio tone in completely unpredictable ways. In addition, the echo-cancelling and noise suppression devices in the call channel treat the single-frequency audio tone as noise or echo, and consequently attempts to filter or suppress it. Various combinations of tones have been tried in an attempt to overcome the aforementioned problems, however none have been found to produce better than about 95% accuracy with respect to a comparison of the injected tone(s) with the frequency and audio level of the looped back tone. Second, there is a settling time associated with the use of single frequency audio tones such that approximately one minute is required to test each voice channel and there are generally hundreds of voice channels associated with each base station. In order to save time, additional test devices in the form of costly specially designed test equipment are needed. However, there is a physical limit to the number of test devices that can be connected to the cellular system at one time.
A second test method, referred to as a Markov test is also used for fault detection on voice channels. It requires a special test mobile station to initiate a “markov call” which requires normal vocoding to be disabled and the normal call processing model to be significantly modified. As a result the Markov test only verifies a portion of the voice channel; the air interface between the network vocoders and the special test mobile station.
A third test method referred to as Perceptual Speech Quality Measurement test, defined in the ITU P.861 “Series P: Telephone Transmission Quality” specification, has been suggested as a alternative to SALT. The purpose of PSQM test is to measure the perceived quality of POTS telephony vocoders in an automated fashion, rather than having to rely on a statistical analysis of responses of human test subjects as is done to Mean Opinion Scores (MOS). In general, the PSQM test works by modeling the way which humans perceive the quality of speech. Test samples are injected into the PSQM model which generates a set of parameters from the test samples. The parameters are then forwarded to a mapping function where they are evaluated and assigned a Mean Opinion Score (MOS). The PSQM test however is not intended for use in a wireless communication environment. Further, even if it were applicable to wireless communication environments, it would require complex and expensive hardware modifications to test equipment currently used for fault detection on voice channels.
A fourth method used for fault detection on a voice channel utilizes a recorded reference speech sample (original recorded speech sample) which is injected into the call path of a first special test mobile station and received by a second special test mobile which records the resulting speech. In the Speech Waveform Matching test, the recorded resulting speech is compared to the original reference speech sample and the degree to which they match is measured. However, the process of vocoding does not necessarily reconstruct a replica that looks like the original recorded speech sample. Instead, vocoding reconstructs a replica that sounds like, to a human listener, the original recorded speech sample. Thus, the degree to which the original reference speech sample and the recorded resulting speech sample, matched, would always be skewed by the very presence of the vocoders in the voice channel path. In addition, the accuracy of the Speech Waveform Matching test is a function of the time it takes to perform the test. A longer recorded resulting speech sample yields a better correlation result than a shorter recorded resulting speech sample. However, the longer recorded resulting speech samples require more time to collect. Further, the special test mobile stations lack sufficient memory capacity to accommodate both the original reference speech sample and the longer recorded resulting speech.